1. Field
Apparatuses and methods consistent with embodiments of the present invention relate to a thermal bimorph probe for use in scanning thermal microscopy, and in particular to a thermal bimorph probe formed so as to twist when heated, thereby isolating the thermal signal from the topographical signal and providing improved thermal and spatial resolution.
2. Description of the Related Art
Scanning thermal microscopy (SThM) is a microscopy technique used in the analysis of thermal, electronic and photonic transport at dimensions approaching the mean free path of phonons and other quasi particles. The scanning thermal microscopy technique uses the atomic force microscope, but includes the additional ability to map thermal properties. An atomic force microscope (AFM) is used for SThM because the microscope maintains a set distance/force to the surface, which is necessary to map thermal properties. Coupling the atomic force microscope with specialized probes has also been used to image other surface properties including electrical properties, magnetic properties and optical properties. The ability to map thermal transport and heat dissipation in nanoscale features is important to the semiconductor industry, photovoltaic industry and other high-end physical material studies.
FIG. 1 illustrates a typical set up for an atomic force microscope 100. The atomic force microscope creates images of surface topography by tracing a probe tip 111 over a sample 140 surface with a constant force. The height of the piezo tube actuator 160 then changes to maintain a constant force between the probe tip 111 and the sample 140. This is used to create the image of the surface of the sample 140. The probe 110 is a disposable part of the atomic force microscope 100 and required to implement any atomic force microscopy or scanning thermal microscopy technique. An unspecialized atomic force microscope probe used for imaging topography is primarily composed of the tip portion 111, a cantilever portion 112 and a body portion or probe chip 113. The tip portion 111 extends from the end of the cantilever portion 112 and ends in a point with a shape often described as hemispherical. Typically tips extend anywhere between 1 to 100 micrometers from the cantilever. The tip radius (or the radius of the hemispherical point) can range from 1 to 100 nm, but are typically between 5 to 20 nm.
In the atomic force microscope 100 shown in FIG. 1, the probe chip 113 of the probe 110 is used for handling the probe 110 and mounting the probe 110 in the microscope 100. The tip 111 is used to sample the surface topography; as such, the spatial resolution of the topography imaging is limited by the diameter of the tip. The deflection of the cantilever portion 112 is proportional to the applied force and is measured by reflecting a laser beam emitted by a laser 120 off the cantilever portion 112 of the probe 110 into a quadrant photodetector 130. In the atomic force microscope 100, the tip/sample force controlling feedback loop is composed of the force modulating piezo tube actuator 160, the force measuring quadrant photodiode 130, and the processor 150. The piezo tube is composed of several piezoelectric crystals actuators that are used to control the position of the sample 140 with respect to the tip 111. In one example, tip remains fixed while the piezo tube is moved. Alternatively, the tip can be moved while the sample remains fixed. The quadrant photodiode 130 is composed of four photodetectors (131, 132, 133 and 134) and is used to monitor the deflection of the cantilever portion 112.
The deflection of the cantilever portion 112 is measured by the position of the laser spot in the quadrant photodetector 130, specifically the output of the top photodetectors (131+132) minus the output from the bottom photodetectors (133+134). Initially, with no force between the sample 140 surface and the tip 111, the deflection signal from the quadrant photodetector 130 is zero and as force is applied and the cantilever portion 112 deflects upward the deflection signal becomes a positive value. A single pixel of the image is obtained when the processor 150 moves the sample 140 via the piezo tube 160 towards the tip 111 such that the cantilever portion 112 reflects the laser spot in the quadrant photodiode 130 to a setpoint deflection value. The topographical pixel value is recorded as the distance the piezo tube moved to reach the setpoint. The setpoint, scan size, and number of pixels are entered by the user into the processor 150 prior to the scan. The scan size is the total area in the X and Y direction that the piezo tube 160 must move during the scan. The scan size and the number of pixels are used to determine the distance the piezo tube 160 must move in the X-Y direction between each pixel. All of the pixels are combined to create an image of the surface topography and output to the processor 150, which can include a feedback controller and user interface. One of ordinary skill in the art will understand that the above explanation of the atomic force microscope is just one example of the atomic force microscope and of the microscopy technique, and has been provided for background purposes only.
The most common form of atomic force microscopy is called “tapping mode” and is similar to that described above, except the probe 110 is vibrated at the cantilever resonance frequency and the monitored signal (proportional to force) is the root mean squared of the “normal” topographical signal previously described. Furthermore, one of ordinary skill in the art will understand that the lateral signal from the quadrant photodetector is composed of the left two photodetector (131, 133) signals minus the right two photodetector (132, 134) signals. The lateral signal is a measure of cantilever twisting and is typically not used in “tapping mode”.
The scanning thermal microscopy technique allows for thermal measurements using thermal bimorph probes. This “thermal bimorph approach” is a simple and cheap technique for implementing scanning thermal microscopy, because it involves using the atomic force microscope quadrant photodiode 130 (shown in FIG. 1) for thermal transduction. This technique theoretically should provide a much better thermal resolution (10−5 K, 2 orders of magnitude lower) than other known approaches.
The thermal bimorph probe is made from two intimately bound materials that have different thermal expansions. When the thermal bimorph probe is heated the difference in thermal expansion causes a bending stress in the material coating the probe 110. The bending stress is a significant drawback of the thermal bimorph approach, as explained below.
In one example, conventional topographical AFM probe may include a reflective layer provided on the cantilever portion to enhance the reflection of the laser. As explained below, the reflective layer has different thermal expansion properties than the material making up the cantilever portion of the probe, which results in the cantilever portion bending during the application of heat and is considered a nuisance to AFM topographical imaging.
FIGS. 2 and 3 illustrate the impact of heat on two types of conventional bimorph probes. FIG. 2 illustrates a so-called “diving board” probe 300. This probe 300 includes a body portion or probe chip 350, a cantilever portion 310 extending from the probe chip 350 and a tip 340 attached to the end of the cantilever portion 310. The cantilever portion 310 is made up of two different materials, a first material 320 (on top) and a second material 330 (on bottom). These two materials typically have different thermal expansion properties. Conventionally, however, the first material was selected to match the thermal expansion properties of the second material, in an attempt to avoid the drawbacks caused by the bend in the cantilever portion of the probe.
FIG. 3 illustrates another type of probe, referred to as a chevron shaped (V-shaped) probe 360. Probe 360 also includes a body portion or probe chip 390, a cantilever portion 370 extending from the body portion 390 and a tip 380 attached to the end of the cantilever portion 370. As in FIG. 2, the cantilever portion 370 of the probe 360 is made up of two materials, first material 375 (on top) and second material 385 (on bottom), that have different thermal expansion properties. The purpose of the probe tip in atomic force microscopy is to probe the “atomic forces” (which are essentially repulsion forces), while in thermal microscopy, a probe tip is used to sample a small area of the surface of the sample. As a result, the probe tip acts as a bottleneck for the movement of heat between the probe and the surface. Therefore, with optimum thermal sensitivity, the spatial resolution of a thermal image is limited by the size of the probe tip. Assuming very sensitive thermal sensing, the size of the tip impacts the resolution of the measurement.
As shown in FIGS. 2 and 3, when heat is applied to probes 300 and 360, the cantilever portions 310 and 370 bend in a “normal” bending manner. This thermal induced “normal” bending interferes with the ability to measure and the topographical signal, as explained with respect to FIG. 4 below, and is thus considered a nuisance to AFM topographical imaging. The heat that is applied to the probes can come from a laser, or from an external heater (or cooler) applied to the probe or to the surface.
FIG. 4 illustrates thermal and topographical signals of a conventional thermal “diving board” probe 300 in a thermal bimorph approach. The problem with using the conventional thermal bimorph probes, however, is that the thermal signal and the topographical signal utilize the same signal transduction channel in the quadrant photodiode 130, namely “normal” direction bending. That is, the topographical deflection signal 430 and thermal deflection signal 420 from the surface lead to the cantilever portion being deflected in the same direction (the top 2 photodiodes (A+B) minus the bottom 2 photodiodes (C+D)) in the quadrant photodiode 130, resulting in interference between the thermal signal and the topographical signal. As a result, the thermal bimorph effect is considered a nuisance, and a significant drawback of the thermal bimorph approach. While attempts have been made to circumvent the bending issue that results in overlapping thermal and topographical signals, previous solutions required complex and high-cost electronic structures and were not considered practically feasible.
Other probes using different thermal sensing mechanisms have also been investigated, including thermocouple probes, thermal-resistive based probes and others. However, with the exception of thermal-resistive based probes, these other approaches are not practically feasible due to high fabrication costs, poor performance or other reasons.
Thermal-resistive based probes involve monitoring the thermally induced changes in the electrical resistance of a circuit embedded in the thermal-resistive probe. Unfortunately, thermal-resistive based probes also have drawbacks. For example, the micro-fabrication process necessary to create either of these commercial thermal-resistive probes is quite expensive, resulting in an average cost of about $150-$300/probe, which is 5-10 times higher than the cost of non-specialized topographical atomic force probes.
While probe cost is the major hindrance to widespread use of the thermal-resistive probes, there are other issues with resistive based scanning thermal microscopy. For example, while these commercial thermal-resistive probes can have fairly good spatial resolution (−30 nanometers (nm)), the thermal resolution is far from optimum at ˜0.1 degrees Kelvin (K). Furthermore, resistive based scanning thermal microscopy requires additional hardware which can also be quite expensive (well over $20,000) and is not commercially available for many scanning probe microscopes. Resistive and other electrical-based scanning thermal microscopy probes can also suffer from cross-talk between thermal and electrical signals when imaging electrical circuits. For at least these reasons, thermal-resistive probe have not achieved widespread success.